Mechanically Robust Component Carrier With Rigid and Flexible Portions

ABSTRACT

A component carrier with a rigid portion, a flexible portion, a cavity defining the flexible portion next to the rigid portion, and at least one step in a transition portion between the rigid portion and the flexible portion in the cavity is disclosed.

TECHNICAL FIELD

Embodiments of the invention relate to a component carrier and a methodof manufacturing a component carrier.

BACKGROUND

Component carriers and electronic components mounted thereon and/orembedded therein are in wide use in electronic products. By employingrigid-flexible component carriers which comprise a rigid portion and aflexible or semi-flexible portion, the advantages of both a rigidcomponent carrier and a flexible component carrier may be combined.Increasing miniaturization and growing product functionalities as wellas a rising number of electronic components to be mounted on thecomponent carriers, such as printed circuit boards, lead to more denselypacked electronic devices, wherein an increasing number of electricalcontacts has to be connected.

Bending a rigid-flex board may cause high mechanical load between arigid portion and a flexible portion of the rigid-flex board and maylimit the lifetime of such component carriers.

SUMMARY

There may be a need for a component carrier with rigid portion andflexible portion which is mechanically reliable even under harshconditions.

According to an exemplary embodiment of the invention, a componentcarrier is provided which comprises a rigid portion, a flexible portion,a cavity delimiting the flexible portion from the rigid portion (ordefining the flexible portion next to the rigid portion, or separatingthe flexible portion from the rigid portion, or defining the flexibleportion versus the rigid portion), and at least one step in a transitionportion between the rigid portion and the flexible portion in thecavity.

According to another exemplary embodiment of the invention, a method ofmanufacturing a component carrier is provided, wherein the methodcomprises forming a stack comprising a plurality of electricallyconductive layer structures and a plurality of electrically insulatinglayer structures, forming a cavity in the stack to thereby delimit aflexible portion and a rigid portion (or to distinguish a flexibleportion from a rigid portion), and forming at least one step in atransition portion between the rigid portion and the flexible portion.

Overview of Embodiments

In the context of the present application, the term “component carrier”may particularly denote any support structure which is capable ofaccommodating one or more components thereon and/or therein forproviding mechanical support and/or electrical connectivity. In otherwords, a component carrier may be configured as a mechanical and/orelectronic carrier for components. In particular, a component carriermay be one of a rigid or flexible printed circuit board, an organicinterposer, and an IC (integrated circuit) substrate. A componentcarrier may also be a hybrid board combining different ones of the abovementioned-types of component carriers.

In the context of the present application, the term “rigid portion” mayparticularly denote a portion of the component carrier which, whenapplying or exerting ordinary forces typically occurring duringoperation of the component carrier, the rigid portion will remainsubstantially undeformed. In other words, the shape of the rigid portionwill not be changed when applying forces during operation of thecomponent carrier.

In the context of the present application, the term “flexible portion”may particularly denote a portion of the component carrier which, uponexerting typical forces occurring during operation of the componentcarrier, will result in an elastic deformation of the flexible portion.The elastic deformation of the flexible portion may be possible to suchan extent that the shape of the entire component carrier may besignificantly influenced by deforming the flexible portion.

In the context of the present application, the term “cavity” mayparticularly denote a recess (such as a blind hole, i.e. a hole with aclosed bottom, or a through-hole, i.e. a hole which extends through anentire body) defining a hollow space of the component carrier. In thecontext of the present application, thick portions of the componentcarrier defining lateral sidewalls of the cavity may form part of therigid portion. In contrast to this, a thinner portion of the componentcarrier also defining a top or bottom surface extending along ahorizontal plane may form part of the flexible portion.

In the context of the present application, the term “step” mayparticularly denote a structural or spatial discontinuity within thecavity in a spatial region in which the rigid portion and the flexibleportion are adjacent to one another. Such a discontinuity may be anadditional material portion modifying a corner of the cavity by forminga step shaped feature. The mentioned discontinuity may however also be aregion of lacking material as compared to an ordinary corner of thecavity which may additionally form a step feature in the transitionportion between flexible portion and rigid portion.

According to an exemplary embodiment of the invention, a rigid-flexcomponent carrier may be provided having a rigid portion and a flexibleportion and a step-like feature within a cavity and between the rigidportion and the flexible portion. When bending the rigid-flex componentcarrier at the flexible portion and/or when applying other types ofmechanical load (for instance when fastening the component carrier on asupport, for instance by a screw), a significant force or mechanicalload may be exerted specifically in the transition portion between therigid portion and the flexible portion in a corner region of the cavity.Conventionally, this transition portion may thus be a region of veryhigh mechanical load being specifically prone to breakage or tonegatively influence the bonding between the layers at least. Whenoperating a rigid-flex board, tensile strain is applied to such a cornerregion of the cavity which may conventionally result in a risk ofdamage. However, the present inventors have surprisingly found thatapplying a compressive load (rather than a tensile load) to such acorner region may significantly reduce the risk of breakage. Withoutwishing to be bound to a specific theory it is presently believed thatapplying such a compressive force to the corner region will result in abreakage only at significantly higher forces compared to the applicationor exertion of tensile forces. In view of this finding, the presentinventors have additionally found that when forming a spatialdiscontinuity in form of a step in the corner region of the cavity, i.e.at the transition between rigid portion and flexible portion, a forceapplied to the rigid-flex component carrier may experience anadvantageous force direction conversion. More specifically, in thepresence of the step, a tensile force may be redirected or converted atleast partially into a compressive force before exerting the latter tothe transition portion of the component carrier. As a surprising result,the rigid-flex component carrier is significantly less prone to failurewhen bending the flexible portion relatively to the rigid portion in thepresence of a step in the cavity. Thus, a component carrier may beprovided which is safely protected against damage thanks to the additionof the step feature in the transition portion of the component carrier.

In the following, further exemplary embodiments of the method and thecomponent carrier will be explained.

A gist of an exemplary embodiment of the invention is the provision of acomponent carrier configured as rigid-flex board without the need ofproviding polyimide or the like and providing a uniform cavity withoutrelease layer and with one or more steps in the cavity which improvesthe mechanical integrity. In particular, such a manufacturingarchitecture provides for a double-sided component carrier (inparticular embodied as printed circuit board, PCB) with bending areawhich can be manufactured with low effort and with reliable quality.This low effort may be achieved in particular by defining a bending areaof the rigid-flex component carrier by milling, in particular by depthcontrolled milling. In particular, it may be possible to produce adouble-sided rigid-flex PCB with a thin FR4 material instead of apolyimide material. Such a manufacturing process involves less effortthan a standard rigid-flex PCB manufacturing process, and has highlyadvantageous bending characteristics. For instance, a two-layer PCB maybe provided with a bending area, wherein one copper layer may beprovided in the bending area and two copper layers may be foreseen in arigid portion or area.

In an embodiment, the rigid portion has a larger thickness than theflexible portion. In other words, the higher rigidity of the rigidportion as compared to the flexible portion may be the result of aselective thinning of a layer stack in the flexible portion only.

In an embodiment, the flexible portion is enclosed at (at least) twoends (in particular edges) by the rigid portion. In such an embodimentit is also possible that the flexible portion is used as an inlayinserted into one or more rigid portions.

In an embodiment, the rigid portion and the flexible portion eachcomprise at least one electrically insulating layer structure. Moreprecisely, both the rigid portion and the flexible portion may comprisea respective stack of at least one electrically insulating layerstructure and/or at least one electrically conductive layer structure.The stack in the flexible portion may be composed of a lower number oflayer structures as compared to the stack in the rigid portion. It isalso possible that one or more of the mentioned layer structuresextend(s) over both the rigid portion and the flexible portion, while atleast one other of the layer structures only extends along the rigidportion. Hence, the rigid portion and the flexible portion may share acommon electrically insulating layer structure. This renders thecomponent carrier mechanically stable, even when bending the flexibleportion.

In an embodiment, the electrically insulating layer structures of therigid portion and of the flexible portion are made of material havingthe same value of the Young modulus, in particular are made of the samematerial. For instance, the electrically conductive layer structuresboth in the rigid portion and in the flexible portion may be made ofcopper, whereas the electrically insulating layer structures in both therigid portion and the flexible portion may comprise resin, if desired incombination with reinforcing structures such as glass fibres or glassspheres. An optional solder mask in the flexible region may however bedifferent than in the rigid region. By using the same materials for boththe rigid portion and the flexible portion, a manufacture of thecomponent carrier with low effort and homogeneous material compositionis accomplished. The latter also has advantages in terms of thermal loadwhich can be kept small with a configuration of the component carrierhaving similar or identical materials in the rigid portion and theflexible portion. The rigidity and the flexibility in the rigid portionand the flexible portion, respectively, may then be the mere result of adifferent thickness in the mentioned portions. However, alternatively,it is also possible to use more flexible materials (such as polyimide)in the flexible portion than in the rigid portion (for instanceprepreg).

In an embodiment, the electrically insulating layer structures of therigid portion and of the flexible portion comprise resin withreinforcing particles, in particular reinforcing glass fibers or glassspheres. The presence of reinforcing structures such as glass fibres areconsidered as an important source for failure upon applying an excessivetensile force to the transition region. It is believed that the glassfibres may break here even in the presence of a relatively small tensileforce (in the absence of a step). However, the presence of the mentionedone or more steps in the transition region may redirect the forcesexerted to the component carrier in particular in the transitionportion. As a result, the tendency of breakage in the transition regionmay be strongly suppressed. In particular, the reinforcing particles maybe prevented from breakage in such a scenario.

In an embodiment, the cavity has a rectangular cross-section withcorners, wherein the step is formed in at least one of the corners, i.e.as an additional feature in the corner. In other words, the step may bea feature being provided in addition to the corner of the cavity. By theprovision of such a step feature, the tendency of a component carrier ofbreaking when a force is exerted to this corner region may be stronglysuppressed. In an embodiment, the step may be precisely formed in thecorner so that the step and the corner may overlap.

In an embodiment, the flexible portion is one of the group consisting ofa fully flexible portion, and a semi-flexible portion. Thus, theflexible portion may be configured so that only a slight bending(semi-flexible property) or a strong or dynamic bending (fully flexibleproperty) is enabled.

In an embodiment, the fully flexible portion comprises or consists of atleast one of the group consisting of polyimide, polyamide, and liquidcrystal polymer (LCP). Other fully flexible materials may be used aswell.

In an embodiment, the semi-flexible portion comprises or consists of atleast one of the group consisting of FR4, and Resin Coated Copper (RCC).FR4 may be a combination of resin (in particular epoxy resin) andreinforcing particles (in particular glass fibres). An RCC can bedenoted as a semifinished product comprising a copper foil covered witha resin.

In an embodiment, a width of the at least one step is at least 20 μm, inparticular at least 50 μm, in a horizontal direction of the plate shapedcomponent carrier. In an embodiment, a height of the at least one stepis at least 20 μm, in particular at least 50 μm, in a vertical directionof the plate shaped component carrier. It has turned out that stepdimensions of the mentioned sizes are appropriate for significantlypreventing the tendency of the component carrier to break in thetransition region in the event of mechanical load. The steps may howeverbe also larger, for instance at least 100 μm.

In an embodiment, the at least one step is formed at least partially bya cured low-flow prepreg layer or a cured no-flow prepreg layer in thetransition portion. In the context of the present application, the term“low-flow material” (sometimes also denoted as “no-flow material”) mayparticularly denote material which has no or only a very limitedtendency to flow during processing under external pressure and elevatedtemperature, in particular during lamination. In particular, low-flowmaterial may have a sufficiently high viscosity, for instance at least5,000 Poise, preferably at least 10,000 Poise, at lamination temperature(for instance 150° C.). For example, when ordinary prepreg is heatedunder pressure, its resin melts (liquefies) and freely flows in anyvoids in the environment. There is a certain period of time during whichthe resin of ordinary prepreg remains fluidic enough to flow freely. Incontrast to this, low-flow material as implemented in accordance withexemplary embodiments of the invention is specifically configured tosuppress or even eliminate flow during lamination, so that the low-flowmaterial substantially rests in place during lamination. However, the“low-flow material” or “no-flow material” may still be at leastpartially uncured when being provided prior to lamination. Such ano-flow prepreg or low-flow prepreg has the tendency of substantiallynot flowing into the region of the step when connecting the no-flowprepreg or low-flow prepreg with other layer structures duringlamination. In the presence of thermal energy and/or pressure, anordinary prepreg may re-melt and the corresponding resin may flow intotiny gaps during lamination. After a corresponding cross-linkingprocedure of the resin is completed, the resin is re-solidified and thenremains spatially fixed. If ordinary prepreg is used for the componentcarrier according to an exemplary embodiment of the invention, careshould be taken to prevent excessive flow of resin into a region (suchas the below described indentation) which should be kept free of resinfor forming the step. However, when using low-flow prepreg or no-flowprepreg, such potential issues are overcome by preventing the flow ofresin into a gap which shall remain free of material for properlydefining the step. Thus, the use of no-flow prepreg or low-flow prepregis highly preferred.

In an embodiment, the at least one step forms a convex protrusion (suchas a circumferentially closed protrusion) extending from at least onecorner of the cavity into the cavity. Such a rectangular protrusion mayhighly efficiently redirect forces exerted to the transition region froma tensile force into a compressive force, thereby efficientlysuppressing the risk of breakage in the corner region of the rigid-flexcomponent carrier.

In an embodiment, the at least one step forms an undercut (such as acircumferentially closed undercut) in at least one corner of the cavity.When such an undercut is formed in the transition region, the undercutis accompanied by a portion of a layer structure extending further intothe cavity and being located below the undercut. It has turned out thateven such a step feature has the capability of resulting in the abovedescribed force redirection with the effect that there is a lowertendency of breakage in the corner region.

In an embodiment, the flexible portion is arranged between differentsections of the rigid portion. For instance, the flexible region may bea central region of the component carrier, whereas two rigid portionsare formed as being connected to respective opposing ends of theflexible region.

In an embodiment, the at least one step is configured as one of thegroup consisting of a single step and a double step. For instance, thecorner of the cavity may be equipped with exactly one step (in additionto the corner itself), which may correspond to a protrusion or anundercut. Alternatively, the corner of the cavity may be provided withtwo steps, which may correspond to two stepped protrusions or twostepped undercuts. In particular a double step or double undercut mayprovide a strong robustness against failure in the transition portion.

In an embodiment, the method comprises forming the cavity by removingmaterial of the stack. This may be accomplished by milling, inparticular by deep milling. In particular by the combination of amilling procedure with a formation of the stack using two constituentsbeing both already fully cured at the time of lamination, removal of aseparated piece of the layer structures can be accomplished without theuse of a release layer.

With the concept of a release layer, which can be implemented in anotherexemplary embodiment of the invention as well, a non-adhesive materialis embedded within a layer stack. The formation of a cavity may then beaccomplished by cutting out a portion of the layer stack (for instanceby milling along a circumferential line) and simply taking out theseparated piece. This simple separation is possible as a result of thenon-adhesive property of the release layer (for instance a layer made ofa waxy material).

In an embodiment, the method comprises defining the at least one step bycorrespondingly positioning a milling tool for removing material of thestack for forming the cavity. Descriptively speaking, by adjusting thelateral position of the milling tool, it is simply possible to definethe position, dimension and type (i.e. protrusion or undercut, singlestep or multiple step, etc.) of the mentioned step. Reference is made toFIG. 7 to FIG. 11.

In an embodiment, the method comprises forming the stack by arranging anuncured layer structure (in particular a low-flow or no-flow uncuredlayer structure) between a first fully cured layer structure and asecond fully cured layer structure, and subsequently curing the uncuredlayer structure. In the context of the present application, the term “atleast partially uncured material” particularly denotes material whichhas the property to at least partially melt or become flowable by theapplication of elevated pressure and/or elevated temperature, and becomefully hardened or cured (and thereby becomes solid) when releasing theapplied elevated pressure and/or elevated temperature. Consequently,applying elevated pressure and/or elevated temperature may causeliquidation of the curable or at least partially uncured material,followed by a hardening (for instance an irreversible hardening in thecase of thermosetting materials, wherein other materials may be used aswell) upon releasing the applied high pressure and/or high temperature.In particular, the “at least partially uncured material” may comprise orconsist of B-stage material and/or A-stage material. By providing thelayer structure from resin, prepreg or any other B-stage material, thelayer structure may re-melt during lamination so that resin (or thelike) may flow for interconnecting the various elements and for closinggaps or voids and may therefore contribute to a stable intrinsicinterconnection within the component carrier under manufacture. Uponconnecting such a structure by the application of pressure and/or heat,i.e. by lamination, only the low-flow prepreg or no-flow prepreg willre-melt slightly and accomplish a local connection. The two fully curedlayer structures will not establish a mutual adhesive connection,allowing to subsequently take out a piece delimited by a circumferentialmilling or cutting line and the direct connection area between the twofully cured layer structures.

Additionally or alternatively to the provision of an uncured layerstructure, it is also possible to use other connection techniques suchas the provision of an adhesive layer (for instance an anisotropicadhesive), a bonding sheet, etc.

In an embodiment, the first fully cured layer structure has a steppedprofile with a central protrusion (for instance made of copper)surrounded by a lateral base (for instance made of a dielectric, whichmay comprise resin) and an indentation (such as a circumferential orannular channel or groove) between the protrusion and the base.Descriptively speaking, the central protrusion may define an area whereno adhesion between the two fully cured layer structures is enabled,whereas the base may define an indirect connection area between thefully cured layer structures via an uncured layer structure describedbelow.

In an embodiment, the (preferably, but not necessarily low-flow orno-flow) uncured layer structure is a patterned layer with a centralrecess extending to laterally encompass the protrusion and theindentation and to accommodate the protrusion. In other words, therecess and the protrusion may match or may be adjusted to one another.Thus, a form closure between the constituents to be connected with oneanother may be established, thereby ensuring a proper mutual positioningof these constituents. The connection between the two fully cured layerstructures may be established exclusively in a region where the uncuredlayer structure is present. In a region of a direct connection betweenthe fully cured layer structures however, intentionally no connection isestablished, which allows formation of the cavity by milling or thelike.

In an embodiment, the method comprises forming the cavity by removingmaterial substantially laterally inside of the indentation. Thanks tothe intentionally poor adhesion between the two fully cured layerstructures at their direct connection surfaces, a corresponding materialpiece can be simply taken out and thereby separated from the rest of thelayer stack.

In an embodiment, the method comprises removing material substantiallylaterally inside of the indentation by cutting substantially around theindentation and taking out a piece of material. This piece of materialmay be defined laterally by a corresponding cutting line (for instanceformed by milling) and horizontally by a non-adhering interface betweenthe first fully cured layer structure and the second fully cured layerstructure. Preferably, such a circumferential cut may be carried out bya milling tool while simultaneously defining the at least one step.

It is possible that one or more components is or are surface mounted onand/or embedded in the component carrier. The at least one component canbe selected from a group consisting of an electrically non-conductiveinlay, an electrically conductive inlay (such as a metal inlay,preferably comprising copper or aluminum), a heat transfer unit (forexample a heat pipe), a light guiding element (for example an opticalwaveguide or a light conductor connection), an electronic component, orcombinations thereof. For example, the component can be an activeelectronic component, a passive electronic component, an electronicchip, a storage device (for instance a DRAM or another data memory), afilter, an integrated circuit, a signal processing component, a powermanagement component, an optoelectronic interface element, a voltageconverter (for example a DC/DC converter or an AC/DC converter), acryptographic component, a transmitter and/or receiver, anelectromechanical transducer, a sensor, an actuator, amicroelectromechanical system (MEMS), a microprocessor, a capacitor, aresistor, an inductance, a battery, a switch, a camera, an antenna, alogic chip, a light guide, and an energy harvesting unit. However, othercomponents may be embedded in the component carrier. For example, amagnetic element can be used as a component. Such a magnetic element maybe a permanent magnetic element (such as a ferromagnetic element, anantiferromagnetic element or a ferrimagnetic element, for instance aferrite base structure) or may be a paramagnetic element. However, thecomponent may also be a further component carrier, for example in aboard-in-board configuration. One or more components may be surfacemounted on the component carrier and/or may be embedded in an interiorthereof. Moreover, also other than the mentioned components may be usedas a component.

In an embodiment, the component carrier comprises a stack of at leastone electrically insulating layer structure and at least oneelectrically conductive layer structure. For example, the componentcarrier may be a laminate of the mentioned electrically insulating layerstructure(s) and electrically conductive layer structure(s), inparticular formed by applying mechanical pressure, if desiredaccompanied or supported by thermal energy. The mentioned stack mayprovide a plate-shaped component carrier capable of providing a largemounting surface for further components and being nevertheless very thinand compact. The term “layer structure” may particularly denote acontinuous layer, a patterned layer or a plurality of non-consecutiveislands within a common plane.

In an embodiment, the component carrier is shaped as a plate. Thiscontributes to the compact design, wherein the component carriernevertheless provides a large basis for mounting components thereon.Furthermore, in particular a naked die as example for an embeddedelectronic component, can be conveniently embedded, thanks to its smallthickness, into a thin plate such as a printed circuit board.

In an embodiment, the component carrier is configured as one of thegroup consisting of a printed circuit board, and a substrate (inparticular an IC substrate).

In the context of the present application, the term “printed circuitboard” (PCB) may particularly denote a component carrier (which may beplate-shaped (i.e. planar), three-dimensionally curved (for instancewhen manufactured using 3D printing) or which may have any other shape)which is formed by laminating several electrically conductive layerstructures with several electrically insulating layer structures, forinstance by applying pressure, if desired accompanied by the supply ofthermal energy. As preferred materials for PCB technology, theelectrically conductive layer structures are made of copper, whereas theelectrically insulating layer structures may comprise resin and/or glassfibers, so-called prepreg or FR4 material. The various electricallyconductive layer structures may be connected to one another in a desiredway by forming through-holes or any other kinds of interconnection (inparticular with an angle between 0° and 90°, preferably perpendicular tothe surface) through the laminate, for instance by laser drilling ormechanical drilling, and by filling them with electrically conductivematerial (in particular copper), thereby forming vias as through-holeconnections. Apart from one or more components which may be embedded ina printed circuit board, a printed circuit board is usually configuredfor accommodating one or more components on one or both opposingsurfaces of the plate-shaped printed circuit board. They may beconnected to the respective main surface by soldering. A dielectric partof a PCB may be composed of resin with reinforcing fibers (such as glassfibers).

In the context of the present application, the term “substrate” mayparticularly denote a small component carrier having substantially thesame size as a component (in particular an electronic component) to bemounted thereon. More specifically, a substrate can be understood as acarrier for electrical connections or electrical networks as well ascomponent carrier comparable to a printed circuit board (PCB), howeverwith a considerably higher density of laterally and/or verticallyarranged connections. Lateral connections are for example conductivepaths, whereas vertical connections may be for example drill holes.These lateral and/or vertical connections are arranged within thesubstrate and can be used to provide electrical and/or mechanicalconnections of housed components or unhoused components (such as baredies), particularly of IC chips, with a printed circuit board orintermediate printed circuit board. Thus, the term “substrate” alsoincludes “IC substrates”. A dielectric part of a substrate may becomposed of resin with reinforcing spheres (such as glass spheres).

In an embodiment, dielectric material of the least one electricallyinsulating layer structure comprises at least one of the groupconsisting of resin (such as reinforced or non-reinforced resins, forinstance epoxy resin or Bismaleimide-Triazine resin, more specificallyFR-4 or FR-5), cyanate ester, polyphenylene derivate, glass (inparticular glass fibers, multi-layer glass, glass-like materials),prepreg material, polyimide, polyamide, liquid crystal polymer (LCP),epoxy-based Build-Up Film, polytetrafluoroethylene (Teflon), a ceramic,and a metal oxide. Reinforcing materials such as webs, fibers orspheres, for example made of glass (multilayer glass) may be used aswell. Although prepreg or FR4 are usually preferred, other materials maybe used as well. For high frequency applications, high-frequencymaterials such as polytetrafluoroethylene, liquid crystal polymer and/orcyanate ester resins may be implemented in the component carrier aselectrically insulating layer structure.

In an embodiment, electrically conductive material of the at least oneelectrically conductive layer structure comprises at least one of thegroup consisting of copper, aluminum, nickel, silver, gold, palladium,or tungsten. Although copper is usually preferred, other materials orcoated versions thereof are possible as well, in particular coated withsupra-conductive material such as graphene.

In an embodiment, the component carrier is a laminate-type body. In suchan embodiment, the semifinished product or the component carrier is acompound of multiple layer structures which are stacked and connectedtogether by applying a pressing force, if desired accompanied by heat.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the component carrier can be better understood withreference to the following drawings. The elements and features in thedrawings are not necessarily to scale, emphasis instead being placedupon clearly illustrating the structures and principles of operation ofthe assemblies.

FIG. 1, FIG. 2, FIG. 3, and FIG. 4 illustrate cross-sectional views ofstructures obtained during manufacturing a component carrier, shown inFIG. 5, according to an exemplary embodiment of the invention.

FIG. 6 is a cross-sectional view of a portion of a component carrieraccording to an exemplary embodiment of the invention.

FIG. 7, FIG. 8, FIG. 9, FIG. 10 and FIG. 11 illustrate cross-sectionalviews of stepped cavity portions of component carriers according toexemplary embodiments of the invention.

FIG. 12 illustrates the result of compressive stress experiments carriedout with FR4 material.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

The illustrations in the drawings are schematically presented. Indifferent drawings, similar or identical elements are provided with thesame reference signs.

The aspects defined above and further aspects of the invention areapparent from the examples of embodiment to be described hereinafter andare explained with reference to these examples of embodiment.

Before, referring to the drawings, exemplary embodiments will bedescribed in further detail, some basic considerations will besummarized based on which exemplary embodiments of the invention havebeen developed.

According to an exemplary embodiment of the invention, a componentcarrier configured as rigid-flex board may be manufactured with acavity, which separates a rigid portion from a flexible portion andwhich comprises one or more steps. Descriptively speaking, the presenceof the at least one or more steps functions as mechanical discontinuityand intentionally disturbs propagation of external mechanical loadwithin the component carrier. Such forces may conventionally result in atendency of breakage of the rigid-flex component carrier. Redirection ormanipulation of such forces (in particular of tensile stress) by the oneor more steps may improve the mechanical robustness of the componentcarrier and the mechanical integrity. Such manipulation may involve achange of a force propagation direction and/or an at least partialconversion of highly destructive tensile stress into less destructivecompressive stress. The latter may be achieved by forming at least onestep in a cavity of the rigid-flex component carrier, because theformation of such a step in a cavity has turned out to result in lessstress, less wear and a higher lifetime of the component carrier.

In an embodiment, it is also possible to manufacture the rigid-flexcomponent carrier without polyimide foil. An exemplary embodimentprovides a double sided plated through hole PCB with a milling area forbending. In this respect, one challenge is that the thickness of the FR4material in the milling area is frequently not stable and, in particularduring the bending process, the conductors may crack in this area. Inorder to overcome this issue, an exemplary embodiment of the inventionprovides a simple method of manufacturing a rigid flex PCB using FR4(i.e. resin with reinforcing glass structures) instead of a polyimidefoil for the purpose of establishing a flex region. Advantageously, astable material thickness may be defined by thin FR4 material in thebending area. In comparison with this and in conventional approaches, ithas not been possible to provide a stable material thickness in thebending area. Further advantageously, exemplary embodiments of theinvention enable the production of a rigid-flex component carrier with acavity (defining a flex region) which can be manufactured precisely andin a simple way. This reduces the complexity of the manufacturingprocess. Hence, the manufacturing process involves reasonable effort,and in particular less effort than a standard rigid-flex PCB process,while simultaneously providing better bending characteristics thanobtainable with conventional methods.

FIG. 1 to FIG. 5 illustrate cross-sectional views of structures obtainedduring manufacturing a component carrier 100, shown in FIG. 5, accordingto an exemplary embodiment of the invention.

Referring to FIG. 1, base materials for the manufacturing process areshown. In particular, a low-flow or no-flow uncured layer structure 118is sandwiched between a first fully cured structure 120 and a secondfully cured structure 122. The fully cured structures 120, 122 havealready been fully cured by cross-linking their resin material as aresult of the application of pressure and/or heat. Consequently, afurther application of pressure and/or heated will not re-melt again thealready fully cured structures 120, 122. Thus, the fully curedstructures 120, 122 are incapable or no longer capable of generating anadhering force for connecting adjacent layer structures. In contrast tothis, uncured layer structure 118 has not yet been fully cured bycross-linking its resin material by the application of pressure and/orheat. Thus, applying pressure and/or heat may re-melt the not yet curedresin material of the uncured layer structure 118 which is thereforecapable of providing an adhesion function with connected layerstructures upon triggering the curing process. For instance, the uncuredlayer structure 118 may be made of a low-flow or no-flow prepreg (i.e.resin, such as epoxy resin, comprising reinforcing particles, such asglass fibers). Such a low-flow or no-flow prepreg has the property thatit will only flow to a very limited extent, if at all, into adjacentgaps during curing (which may be triggered by pressure and/or heating).Also liquid dielectrics may be used.

The first fully cured layer structure 120 may be a core, whereas thesecond fully cured layer structure 122 may be a thinner core (i.e.thinner than the core constituting the first fully cured layer structure120). Both of the fully cured layer structures 120, 122 may be composedof a central FR4 layer as electrically insulating layer structure 106covered on both opposing main surfaces thereof by a respective copperfoil as electrically conductive layer structure 104. A thickness of theelectrically insulating layer structure 106 of the first fully curedlayer structure 120 may be larger than a thickness of the electricallyinsulating layer structure 106 of the second fully cured layer structure122. The first fully cured layer structure 120 may also be configured asa multi-layer array.

Referring to FIG. 2, preparation of the base materials shown in FIG. 1before lay-up is illustrated.

As shown, the first fully cured layer structure 120 is provided with astepped profile with a central protrusion 125 surrounded by a lateralbase 126 and an indentation 130 in the base 126. This can beaccomplished by patterning the upper electrically conductive layerstructure 104 of the first fully cured layer structure 120. Theindentation 130 may be formed as a groove or channel in the electricallyinsulating layer structure 106 of the first fully cured layer structure120 along a pre-determined Rigid-flex transition line. The indentation130 may be formed to laterally surround the protrusion 125 and willlater serve for defining formation of a cavity 108. For forming theindentation 130, it is possible to remove the FR4 material of theelectrically insulating layer structure 106 of the first fully curedlayer structure 120 at one side except the region defined by the inlaycopper of protrusion 125 in the later bending area of the rigid-flexcomponent carrier 100 to be manufactured. Dielectric material may bemechanically, physically or chemically removed, copper may be etched. Itis also possible to form the indentation 130 by carrying out a pre-deepmilling procedure in the thick FR4 material of the electricallyinsulating layer structure 106 of the first fully cured layer structure120.

Furthermore, the low-flow or no-flow uncured layer structure 118 ispatterned to form a central recess 128 matching to the protrusion 125.The central recess 128 is positioned and dimensioned to accommodate theprotrusion 125 and to be aligned with the indentation 130. Formation ofthe central recess 128 in the low-flow or no-flow uncured layerstructure 118 may be accomplished, for example, by milling, punching orlaser cutting the low-flow or no-flow uncured layer structure 118selectively. Thus, the upper copper foil of the first fully cured layerstructure 120 is patterned for forming the protrusion 125 providing aform closure with the recessed at least partially uncured low-flow orno-flow layer structure 118. The thickness of the upper electricallyconductive layer structure 104 of the first fully cured layer structure120 forming the protrusion 125 on the one hand and the thickness of theat least partially uncured low-flow or no-flow prepreg layer structure118 may be the same or may be similar so as to obtain a verticalalignment.

Beyond this, the second fully cured layer structure 122 may be processedfor removing the lower electrically conductive layer structure 104, forinstance by carrying out a one-sided etching procedure. In other words,the lower copper foil may be removed from the second fully cured layerstructure 122. When implementing a release layer (i.e. a layer made ofmaterial having poor adhesion properties with regard to surroundingmaterial), a sequential build up is possible as well.

The indentations 130 or slits serve for spatially delimiting the cavity108 to be formed. Descriptively speaking, the indentations 130 support amilling tool 116 (see FIG. 4) to properly spatially define a flex region110 of the component carrier 100 to be formed. As can be taken from FIG.2, a gap “d” is defined between an interior side wall of the uncuredlow-flow or no-flow prepreg layer structure 118 and an adjacent sidewall of the upper electrically conductive layer structure 104 of thefirst fully cured layer structure 120. Preferably, the dimension of gap“d” may be selected to be identical or substantially identical to ahorizontal extension “I” of the indentation 130. More precisely, itshould be mentioned that gap “d” denotes the corresponding dimensionafter connection of the shown layers (which means that, during cutting,the flow of material during lamination should be taken into account).More specifically, the exterior side wall of the indentation 130 may bein alignment with the interior side wall of the uncured low-flow orno-flow prepreg layer structure 118. Furthermore, the interior side wallof the indentation 130 may be in alignment with the sidewall of theupper electrically conductive layer structure 104 of the first fullycured layer structure 120. This geometry advantageously contributes to aproper formation of protrusion 124 and step 114 shown in FIG. 5.

Referring to FIG. 3, an interconnected stack 102 is formed by connectingthe low-flow or no-flow uncured layer structure 118 between the firstfully cured layer structure 120 and the second fully cured layerstructure 122 by lamination, i.e. the application of pressure and/orheat. As a result, only the electrically insulating uncured material ofthe low-flow or no-flow uncured layer structure 118 may re-melt, carryout cross-linking and may subsequently re-solidify. As a result, anadhesion force is created exclusively at the direct interfaces betweenlayer structure 118 and the directly connected layer structures 120,122. Since layer structure 118 is made of low-flow or no-flow material,this material will not flow or will not flow significantly intoindentation 130, thereby advantageously keeping indentation 130 open andthereby simplifying the formation of the cavity 108 in a later process.For ensuring that material of layer structure 118 does not completelyfill indentation 130 during lamination, the recess 128 formed in layerstructure 118 according to FIG. 2 may be made sufficiently large. Incontrast to this, no adhesion force is provided by the fully cured layerstructures 120, 122 where they directly contact each another, since theyare incapable of re-melting during lamination. As a result, no adhesiveconnection will be formed between the fully cured layer structures 120,122 in a region corresponding to the upper main surface of theprotrusion 125.

As can be taken from the cross-sectional view of FIG. 3, the recesseduncured electrically insulating layer structure 118 is configured sothat prepreg material is safely prevented from flowing into theindentations 130 during lamination. The distance or gap “d” may bemaintained unfilled with the previously uncured (low-flow or) no-flowprepreg layer structure 118 being cured during lamination, since inparticular no-flow prepreg has the property of performing substantiallyno-flow during curing. However, the distance or gap “d” may bealternatively partially or entirely filled with the previously uncuredlow-flow (or no-flow) prepreg layer structure 118 being cured duringlamination, since in particular low-flow prepreg has the property ofperforming a certain (however relatively small) flow during curing.

Referring to FIG. 4, material is removed to thereby form a cavity 108 inthe stack 102 to delimit a flexible portion 110 defined by the cavity108 from a rigid portion 112. The flexible portion 110 corresponds tothe portion of the cavity 108, whereas the rigid portion 112 correspondsto thicker portions of the stack 102 around the cavity 108. Forming thecavity 108 may be accomplished by removing material of the stack 102 bymilling using a milling tool 116 (indicated schematically in FIG. 4).During the milling, the lateral position of the milling tool 116 may becontrolled so that steps (see reference numeral 114 in FIG. 5) aredefined. The corresponding spatial adjustability of the position of themilling tool 116 is indicated by double arrows 164. Forming the cavity108 may hence be accomplished by removing material substantiallylaterally inside of the indentation 130 by cutting the stack 102, from abottom side thereof, substantially around the indentation 130.Thereafter, a non-adhering piece 132 of material surrounded by acorresponding cutting line may be simply taken out of the stack 102,thereby obtaining the cavity 108. The piece 132 does not adherecircumferentially, since it is circumferentially separated from the restof the layer structure shown in FIG. 3 by milling. Furthermore, thepiece 132 does not adhere at its top surface which corresponds to theupper main surface of the protrusion 125, since it does not comprisematerial of the (meanwhile cured) layer structure 118. Since the uppersurface of the separation area delimiting the piece 132 is formed by theinterface between the upper electrically conductive layer structure 104of the former first fully cured layer structure 120 and the electricallyinsulating layer structure 106 of the former second fully cured layerstructure 122, the lamination has not caused an adhesion there.

As further shown in FIG. 4, a finishing procedure may be carried out byforming a first solder mask 171 on a portion of exposed electricallyconductive surfaces of the obtained layer structure in the rigid portion112, while a second solder mask 173 may be formed on an upper portion ofexposed electrically conductive surfaces of the obtained layer structurein the bending portion or flexible portion 110.

After having taken out the piece 132 and after formation of the soldermasks 171, 173, the component carrier 100 shown in FIG. 5 is obtained.

Referring to FIG. 5, the PCB manufacturing process may be finished bydefining circumferential step 114 by correspondingly positioning themilling tool 116 for removing material of the stack 102 for forming thecavity 108 and by taking out the correspondingly formed piece 132. Thestep 114 is formed in a transition portion between the rigid portion 112and the flexible portion 110 (which may, more precisely, have theproperties of a semi-flexible portion) in a corner of the cavity 108. Aslight flow of low-flow prepreg or no flow prepreg may occur.

As a result, the component carrier 100 shown in FIG. 5 is obtained whichcomprises the exterior rigid portion 112, the central flexible portion110 and the cavity 108 delimiting the flexible portion 110 from therigid portion 112. In other words, the flexible portion 110 is arrangedbetween or is enclosed by different sections of the rigid portion 112.The step 114 in a transition portion between the rigid portion 112 andthe flexible portion 110 in corners of the cavity 108 improves themechanical integrity, as described below in further detail. While therigid portion 112 and the flexible portion 110 comprise substantiallythe same materials (predominantly copper, resin and glass fibers), therigid portion 112 is rendered rigid by providing it with a largervertical thickness than the flexible portion 110. The latter is renderedflexible in view of its small thickness. As can be taken from FIG. 5 aswell, the rigid portion 112 and the flexible portion 110 share a commoncontinuous electrically insulating layer structure 106 which correspondsto the original electrically insulating layer structure 106 of thesecond fully cured layer structure 122.

As shown in FIG. 5, the cavity 108 has a rectangular cross-section withcorners in which the step 114 is formed as a convex protrusion 124. Moreprecisely, the step 114 is formed by the meanwhile cured low-flowprepreg layer structure 118 in the transition portion.

Still referring to FIG. 5, the intentional spatial displacement of themilling tool 116 defines properties of the steps 114 which are embodiedas protrusions 124 in the corners of the formed cavity 108 in theembodiment of FIG. 5. The presence of the steps 114 has a highlypositive impact on the mechanical integrity of the component carrier 100even in the presence of bending forces or other tension forces exertedto the component carrier 100 during operation or handling. As can betaken from reference numerals 153, 155, an exerted force (see referencenumeral 153) may be manipulated or redirected by the step 114 (seereference numeral 155). For instance, tensile forces may be at leastpartially transferred into compressive forces. The capability of thecomponent carrier 100 to deal with compressive forces is much morepronounced than the capability of the component carrier 100 to deal withtensile forces. For instance, such forces may be exerted to thecomponent carrier 100 during screwing the component carrier 100 toanother electronic member, etc. Whichever theoretical explanation may begiven, it has turned out that the presence of the step 114 improves themechanical integrity of the component carrier 100.

In the corner region or transition region of the component carrier 100,the exerted force may be a maximum. However, the breakage force in thecorner region may be very small in the absence of the step 114. Inparticular already quite small tensile forces may cause a breakage (inparticular of glass fibres) of the component carrier 100. By thepresence of the step 114, the force limit of failure is significantlyincreased. The mechanical integrity of the component carrier 100 cantherefore be significantly improved. Without wishing to be bound to aspecific theory it is presently believed that when forming a spatialdiscontinuity in form of step 114 in the corner region of the cavity108, a force (see arrow 153 in FIG. 5) applied to the rigid-flexcomponent carrier 100 may experience an advantageous redirection of theforce direction at or by the step 114 (see arrow 155 in FIG. 5), inparticular at least partially from a vertical force direction to ahorizontal force direction.

Again referring to FIG. 3, three scenarios will be discussed in thefollowing according to which the step 114 is formed by a protrusion 124or an undercut 134.

As indicated with reference numeral 197 in FIG. 3, a first scenario maycorrespond to a position of the milling tool 116 in which the millingtool 116 is positioned laterally outwardly from the indentation 130. Inthis embodiment, a configuration as shown in FIG. 5 can be obtained inwhich the step 114 is formed by protrusion 124, since a relatively largeamount of material of the stack 102 is removed.

As indicated with reference numeral 198 in FIG. 3, a second scenario maycorrespond to a situation in which gap “d” is (here partially) filledwith material of the prepreg layer structure 118 during lamination. Alsoin the second scenario a step 114 is obtained which is defined by aprotrusion 124.

As indicated with reference numeral 199 in FIG. 3, a third scenario maycorrespond to a position of the milling tool 116 in which the millingtool 116 is positioned laterally inwardly from an exterior side wall ofthe indentation 130. In this embodiment, a configuration as shown inFIG. 7 (described below in further detail) can be obtained in which thestep 114 is formed by an undercut 134, since a relatively small amountof material of the stack 102 is removed.

FIG. 6 is a cross-sectional view of a portion of a component carrier 100according to an exemplary embodiment of the invention. FIG. 6 showsanother cross-sectional view of a component carrier 100 according to anexemplary embodiment of the invention in which the shape of the step 114deviates from a rectangle. The shape of the step 114 is more complexaccording to FIG. 6. FIG. 6 illustrates a multilayer structure in thecenter with several flex layers thereon.

FIG. 7 illustrates a cross-sectional view of a stepped cavity portion ofa component carrier 100 according to another exemplary embodiment of theinvention. According to FIG. 7, the step 114 forms an undercut 134 in acorner of the cavity 108. For example, the width “w” of the undercut 134defining the step 114 may be 20 μm in a horizontal direction. The height“h” of the undercut 134 defining the step 114 may be for example 20 μmin a vertical direction.

An undercut 134 is formed by the presence of the step 114 according tothe embodiment of FIG. 7. Similarly, the direction of an external forcecan be redirected by the step 114 for strengthening the robustness ofthe component carrier 100 against failure in the presence of externalload.

A difference between the cavity portion shown in FIG. 8 and the cavityportion shown in FIG. 7 is that, according to FIG. 8, a double undercut134 a, 134 b is formed corresponds to a double step 114, 114. Undercut134 a is located directly adjacent to an upper surface 175 of the cavity108. Undercut 134 b is located vertically between undercut 134 a and astraight interior side wall 177 of the rigid portion 112. The upperundercut 134 a is delimited by an end section of layer structure 118which protrudes laterally beyond an end section of stack 102 delimitingthe lower undercut 134 b.

A difference between the double undercut 134 a, 134 b shown in FIG. 9and the double undercut 134 a, 134 b shown in FIG. 8 is that, accordingto FIG. 9, the lower undercut 134 b is delimited by an end section ofstack 102 which protrudes laterally beyond an end section of layerstructure 118 delimiting the upper undercut 134 a.

FIG. 10 illustrates a cross-sectional view of a stepped cavity portionof a component carrier 100 according to an exemplary embodiment of theinvention. The embodiment of FIG. 10 shows a step 114 embodied as aprotrusion. For example, a width “w” of the step 114 may be 30 μm in ahorizontal direction. A height “h” of the step 114 may be for example 20μm in a vertical direction.

A difference between the cavity portion shown in FIG. 11 and the cavityportion shown in FIG. 10 is that, according to FIG. 11, a doubleprotrusion 124 a, 124 b is formed corresponds to a double step 114, 114.Protrusion 124 a is located directly adjacent to upper surface 175 ofthe cavity 108. Protrusion 124 b is located vertically betweenprotrusion 124 a and straight interior side wall 177 of the rigidportion 112. The upper protrusion 124 a is delimited by an end sectionof layer structure 118 which protrudes laterally beyond an end sectionof stack 102 delimiting the lower protrusion 124 b.

The various embodiments shown in FIG. 7 to FIG. 11 can be manufacturedby adjusting design parameters such as material of layer structure 118(in particular no-flow or low-flow or ordinary prepreg to define theposition and extension of layer structure 118), positioning of millingtool 116 (compare reference numerals 197, 199 in FIG. 3), dimension andrelative position of indentation 130 (compare “d” and “I” in FIG. 2),etc.

FIG. 12 illustrates the result of compressive stress experiments on FR4material (comprising resin and glass fibers). More specifically, diagram200 of FIG. 12 has an abscissa 202 along which nominal strain applied todifferent FR4 samples is plotted in percent. Along an ordinate 204, theresulting tensile stress is plotted in MPa. The tensile strain isbetween 1.4% and 1.7%, whereas compressive strain is about 4%.

It should be noted that the term “comprising” does not exclude otherelements or steps and the “a” or “an” does not exclude a plurality. Alsoelements described in association with different embodiments may becombined.

Implementation of the invention is not limited to the preferredembodiments shown in the figures and described above. Instead, amultiplicity of variants are possible which use the solutions shown andthe principle according to the invention even in the case offundamentally different embodiments.

1. A component carrier, comprising: a rigid portion; a flexible portion;a cavity defining the flexible portion next to the rigid portion,wherein the cavity has a substantially rectangular cross-section withcorners; and at least one step in a transition portion between the rigidportion and the flexible portion in the cavity; wherein the at least onestep is formed only in a corner region of at least one of the corners.2. The component carrier according to claim 1, wherein the rigid portionhas a larger thickness than the flexible portion.
 3. The componentcarrier according to claim 1, wherein the rigid portion and the flexibleportion each comprise at least one electrically insulating layerstructure and share a common electrically insulating layer structure. 4.The component carrier according to claim 3, wherein the at least oneelectrically insulating layer structure of the rigid portion and the atleast one electrically insulating layer structure of the flexibleportion are made of material having the same value of the Young modulus.5. The component carrier according to claim 3, wherein the at least oneelectrically insulating layer structure of the rigid portion and the atleast one electrically insulating layer structure of the flexibleportion comprise resin with reinforcing glass particles.
 6. (canceled)7. The component carrier according to claim 1, wherein the flexibleportion is one of the group consisting of a fully flexible portion, anda semi-flexible portion.
 8. The component carrier according to claim 7,wherein the fully flexible portion comprises or consists of at least oneof the group consisting of polyimide, polyamide, and liquid crystalpolymer.
 9. The component carrier according to claim 7, wherein thesemi-flexible portion comprises or consists of at least one of the groupconsisting of FR4, and Resin Coated Copper.
 10. The component carrieraccording to claim 1, wherein a width of the at least one step is atleast 20 μm in a horizontal direction.
 11. The component carrieraccording to claim 1, further comprising at least one of the followingfeatures: wherein a height of the at least one step is at least 20 μm ina vertical direction; wherein the at least one step is formed at leastpartially by a cured low-flow prepreg layer or a cured no-flow prepreglayer in the transition portion; wherein the at least one step forms aconvex protrusion extending from at least one corner of the cavity intothe cavity; wherein the at least one step forms an undercut in at leastone corner of the cavity; wherein the flexible portion is arrangedbetween different sections of the rigid portion; wherein the at leastone step is configured as one of the group consisting of a single step,and a double step.
 12. The component carrier according to claim 1,further comprising at least one of the following features: at least oneelectrically conductive layer structure comprising at least one of thegroup consisting of copper, aluminum, nickel, silver, gold, palladium,and tungsten, any of the mentioned materials being optionally coatedwith supra-conductive material such as graphene; at least oneelectrically insulating layer structure comprising at least one of thegroup consisting of resin, reinforced or non-reinforced resin, epoxyresin or Bismaleimide-Triazine resin, FR-4, FR-5, cyanate ester,polyphenylene derivate, glass, prepreg material, polyimide, polyamide,liquid crystal polymer, epoxy-based Build-Up Film,polytetrafluoroethylene, a ceramic, and a metal oxide; at least onecomponent mounted on or embedded in the component carrier, wherein theat least one component is selected from a group consisting of anelectronic component, an electrically non-conductive and/or electricallyconductive inlay, a heat transfer unit, an energy harvesting unit, anactive electronic component, a passive electronic component, anelectronic chip, a storage device, a filter, an integrated circuit, asignal processing component, a power management component, anoptoelectronic interface element, a voltage converter, a cryptographiccomponent, a transmitter and/or receiver, an electromechanicaltransducer, an actuator, a microelectromechanical system, amicroprocessor, a capacitor, a resistor, an inductance, an accumulator,a switch, a camera, an antenna, a magnetic element, a light guidingelement, a further component carrier and a logic chip; the componentcarrier is shaped as a plate; the component carrier is configured as aprinted circuit board, or a substrate.
 13. A method of manufacturing acomponent carrier, the method, comprising: forming a stack having aplurality of electrically conductive layer structures and a plurality ofelectrically insulating layer structures; forming a cavity in the stackto thereby delimit a flexible portion from a rigid portion, wherein thecavity has a substantially rectangular cross-section with corners; andforming at least one step in the cavity in a transition portion betweenthe rigid portion and the flexible portion; wherein the at least onestep is formed only in a corner region of at least one of the corners.14. The method according to claim 13, further comprising: forming thecavity by removing material of the stack by milling.
 15. The methodaccording to claim 13, further comprising: defining the at least onestep by correspondingly positioning a milling tool for removing materialof the stack for forming the cavity.
 16. The method according to claim13, wherein forming the stack is accomplished by: arranging an uncuredlayer structure between a first fully cured layer structure and a secondfully cured layer structure; and thereafter curing the uncured layerstructure.
 17. The method according to claim 16, wherein the first fullycured layer structure has a stepped profile with a central protrusionsurrounded by a lateral base and an indentation between the centralprotrusion and the base.
 18. The method according to claim 17, whereinthe uncured layer structure is a patterned layer with a central recessextending to laterally surround the protrusion and the indentation andto accommodate the protrusion.
 19. The method according to claim 17,further comprising: forming the cavity by removing materialsubstantially laterally inside of the indentation.
 20. The methodaccording to claim 19, wherein removing material substantially laterallyinside of the indentation comprises cutting substantially around theindentation and taking out a piece of material defined laterally by acorresponding cutting line and horizontally by a direct interfacebetween the first fully cured layer structure and the second fully curedlayer structure.